The present disclosure relates generally to a well bore servicing fluid, and more particularly to the viscosification of a well bore servicing fluid using polymers comprising betaine units.
Natural resources such as gas and oil can be recovered from subterranean formations using well-known techniques. The processes for preparing a well bore for the recovery of such resources often employ various well bore servicing fluids. For example, drilling fluids or muds are typically circulated through well bores as they are drilled into the formation. During the drilling process, the drill bit generates drill cuttings that include small pieces of shale and rock. The drilling fluid carries the drill cuttings in a return flow stream back to the well drilling platform. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run into the well bore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through an annulus, which is located between the exterior of the pipe and the walls of the well bore.
Another fluid known as a gravel packing fluid having relatively large grained sand, e.g., gravel, suspended therein also may be utilized to prevent migration of smaller grained sand from the subterranean formation into the well bore and to maintain the integrity of the formation. In particular, a permeable screen may be placed against the face of the subterranean formation, followed by pumping the gravel packing fluid into the annulus of the well bore such that gravel becomes packed against the exterior of the screen. In addition, a cement slurry may be pumped into the well bore during a primary cementing process in which the cement slurry is placed in the annulus of the well bore and permitted to set into a hard mass (e.g., sheath) to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, e.g., completion and work over operations, may also be performed using cement slurries.
Yet another fluid, known as a fracturing fluid, is often used to fracture the subterranean formation. The fracturing fluid is pumped into the well bore at a rate and a pressure sufficient to form fractures that extend into the subterranean formation, providing additional pathways through which fluids being produced can flow into the well bores. The fracturing fluid is usually a water-based fluid containing a gelling agent, e.g., a polymeric material that absorbs water and forms a gel as it undergoes hydration. The gelling agent serves to increase the viscosity of the fracturing fluid. The fracturing fluid also typically includes particulate matter known as a proppant, e.g., graded sand, bauxite, or resin coated sand, may be suspended in the fracturing fluid. The proppant becomes deposited into the fractures and thus holds the fractures open after the pressure exerted on the fracturing fluid has been released.
Well bore servicing fluids often include rheology-modifying agents, such as viscosification agents. Viscosification agents can reduce or prevent well known problems associated with, for example, thermal thinning of fluids as they pass down the well bore. An example of such problems includes the inability of drilling fluid to suspend drill cuttings therein as it flows back to the surface. Thus, the drill cuttings may settle out of the drilling fluid and become deposited in undesired locations in the well bore. Furthermore, those fluids containing particles such as a cement slurry, a gravel packing fluid, and a fracturing fluid may experience settling of the particles as the fluids are pumped down the well bore. As a result, the particles are not transported to their proper locations in the well bore. Further, in the absence of such particles, the density of the fluids may drop to a level at which they are incapable of withstanding relatively high fluid pressures downhole, particularly in the case of a high density cement slurry. A need therefore exists for maintaining the viscosity of fluids as they are passed into a well bore so as to reduce or prevent the unwanted settling of materials in those fluids.
Polymers are frequently used as viscosification agents. Polymers used in water-based fluids include acrylamide or methacrylamide polymers, generally acrylamide/acrylate polymers obtained by copolymerization of acrylamide and acrylate or by hydrolysis of polyacrylamide. However, these polymers are sensitive to operating and use conditions. For example, a high shear gradient or a high temperature at the bottom of the well can result in a decrease in viscosifying power. Furthermore, they are sensitive to media of high ionic strength as a result of the presence of carboxylate or sulphonate groups, which also result in a loss of their viscosifying power, even precipitation of the molecules in media highly concentrated in divalent ions.
Polyampholytes carrying both positive charges and negative charges, part of which are formed by water-soluble zwitterionic polymers, can be adapted for applications in saline medium. They are generally more soluble and exhibit greater viscosities in saline medium than in deionized water. The reference is then to antipolyelectrolyte behaviour for this type of polymer. However, they may exhibit an inadequate resistance to the salinity of the aqueous media to be treated or to the shear and temperature conditions of use in the said media.
Polymers comprising a betaine group are also known. The document U.S. Pat. No. 4,788,247 describes terpolymers of units deriving from a sulphobetaine, of units deriving from acrylamide (AM) and of hydrophobic units deriving from an ethoxylated alkyl acrylate. The polymerization is carried out in the presence of large amounts of surfactant (SDS) not making possible the production of groups of the hydrophobic units. Furthermore, the polymerization proves to be difficult to reproduce. The terpolymers exhibit thickening effects on saline media.
The documents U.S. Pat. No. 4,607,076, U.S. Pat. No. 5,153,289, U.S. Pat. No. 4,585,846, U.S. Pat. No. 4,822,847 and/or U.S. Pat. No. 4,708,998 describe polymers of units deriving from a sulphobetaine and of units deriving from vinylpyrrolidone (VP). Many or all of these units are hydrophilic units. The polymers exhibit thickening effects on saline media.
The document U.S. Pat. No. 6,284,854 describes polymers formed of 10 to 50 mol % of units deriving from sulphobetaines and 50 to 90 mol % of hydrophobic units. These polymers are used as biocompatible coatings. The polymerizations are carried out in solution, which does not make possible the production of groups of the hydrophobic units.
The document WO 99/03895 describes a process for the micellar polymerization of certain monomers. The process comprises the preliminary preparation of an initial charge comprising hydrophilic monomers and micelles of hydrophobic monomers. Then a solution of initiator and another solution comprising other hydrophilic monomers and micelles of hydrophobic monomers are added continuously.
WO 01/04201 describes in particular polymers of acrylamide and of sulphobetaines prepared by inverse polymerization and of high molecular weight as additives in the paper industry.
WO 00/01746 describes betaine/acrylamide copolymers with a molar mass limited at most to 2 000 000/3 000 000 and comprising a maximum content of betaine monomer of 6%.
There still exists a need for improved viscosification agents for use as well bore-servicing fluids that exhibit one or more properties, such as: good stability at a relatively high ionic strength and/or good stability in a relatively saline medium, such as a brine; good thickening power for media comprising a relatively high ionic strength, such as saline media, including highly saline media; good hold of the stability and/or thickening at relatively high temperature; and/or a thickening power at low contents of polymer.